Field of the Invention
The present invention is related to an apparatus and a method which can record and/or reproduce information on an optical information recording medium by using holography.
Description of the Related Art
Through a BLU-RAY DISC™ specification using a blue-violet semiconductor laser, at present, commercialization of an optical disk having recording density of about 100 GB is presently performed. From now on, large capacity greater than 500 GB is expected from optical disk recording. However, for realizing the above-described ultra-high density by using an optical disk, a high-density technology is needed through another method which is different from a high-density technology using the conventional short wavelength and high numerical aperture of an objective lens to form pits as an implementation of a digitally encoded data stream.
Holographic Data Storage System (HDSS) has been considered as one of promising candidates for future Optical Data Storage (ODS) systems. In this approach, holograms are recorded as an interference pattern inside a photo sensitive medium by using a signal beam and a reference beam. The signal beam is modulated two-dimensionally typically by a spatial light modulator (SLM) and converged in a medium when Fourier Transform recording geometry is employed. The converging signal beam is overlapped with the reference beam in the medium. Thus, an image bearing interference patterns is generated. The interference pattern is recorded in the photo sensitive medium as a hologram. High density recording is feasible in this approach. For example, in angular multiplexing, HDSS can records numerous holograms in the same space by changing the reference beam angle. High data transfer rate is feasible because millions of bits are recorded and reproduced at once by SLM and camera. HDSS can record an encoded data page with a few million pixels with a single light pulse. Furthermore, hundreds of data pages can be multiplexed at the same location in the media. Thus, HDSS has a high possibility of becoming a Post BD system.
However, tolerances of HDSS to physical perturbation such as mechanical instability need to be stringent in return for the associated high density recording. Especially, the tolerances of HDSS using angularly multiplexed recording will be stringent because of the narrow Bragg selectivity associated with the utilized off-axis optical configuration and thick volume recording materials.